1. Field of the Invention
The present invention relates to a power transmission system for a four wheel drive motor vehicle having an inter-axle differential gear mechanism, and more particularly to a locking mechanism for the inter-axle differential gear mechanism of such four wheel drive motor vehicle
2. Description of the Prior Art
The U.S. Pat. No. 4,541,503 issued on Sept. 17, 1985 to H. Akutagawa et al discloses a four wheel drive motor vehicle having an inter-axle differential gear mechanism in addition to inter-wheel differential gear mechanisms provided in respective ones of the front and rear wheel axles. In the motor vehicle disclosed by the U.S. patent, the engine is located in the front part of the vehicle with the output shaft extending in the transverse direction of the vehicle A transmission is provided at one axial end of the engine and connected with the output shaft of the engine to be driven thereby. The transmission has an output member connected with an input member of the inter-axle differential gear mechanism which has two output members, one being connected with the front wheel axle through a front inter-wheel differential gear mechanism and the other with the rear wheel axle through a transfer gear and a rear inter-wheel differential gear mechanism.
The arrangement disclosed by the U.S. patent provides a compact structure in that the inter-axle differential gear mechanism is located on the front wheel axle coaxially with the front inter-wheel differential gear mechanism. The inter-wheel differential gear mechanism is located at a side of the inter-axle differential gear mechanism adjacent to the engine and the transfer gear is located at the opposite side of the inter-wheel differential gear mechanism with respect to the inter-axle differential gear mechanism. Further, the inter-axle differential gear mechanism and the front inter-wheel differential gear mechanism are housed in the clutch housing in which the clutch is provided.
In the driving system disclosed by the U.S. patent, the inter-axle differential gear mechanism is provided with a differential lock mechanism for restricting the operation of the inter-axle differential gear mechanism. For the purpose, the input member of the inter-axle differential gear mechanism has a spline shaft portion which is formed with externally threaded spline teeth and the transfer gear which is connected with one of the output members of the inter-axle differential gear mechanism also has a spline shaft portion which is coaxial with the spline shaft portion on the input member and formed with externally threaded spline teeth aligned with the spline teeth on the spline portion on the input member. A sleeve member having internally threaded splines is provided on the spline shaft portions for axial slidable movement between an unlock position wherein the splines in the sleeve are disengaged from the splines on one of the spline shafts and a lock position wherein the splines in the spline shafts are engaged with the splines in the sleeve so that the input member of the inter-axle differential gear mechanism is directly connected with one output member.
The British patent No. 1,252,753 also discloses a motor vehicle four wheel drive system which has an inter-axle differential gear mechanism provided with a multiple disc type clutch device for controlling the differential action of the differential gear mechanism. The multiple disc type clutch device used in the British patent is considered as being advantageous in that the engagement and disengagement can be controlled quickly and precisely. This type of clutch can be readily used for automatic locking control of the inter-axle differential gear mechanism
The U.S. Pat. No. 3,400,777 issued on Sept. 10, 1968 to C. Hill and the U.S. Pat. No. 3,492,890 issued on Feb. 3, 1970 to C. Hill et al also disclose four wheel drive motor vehicles having multiple disc type clutches for locking the inter-axle differential gear mechanisms.
In view of the advantages of the multiple disc type clutch to be used in the locking device for the inter-axle differential gear mechanism, the spline type locking device for the inter-axle differential gear mechanism in the U.S. patent to H. Akutagawa et al may be replaced by the multiple disc type clutch. It should however be noted that there will be inconveniencies in adopting the multiple disc type clutch in the arrangement of Akutagawa et al because this type of clutch is of a relatively large diameter so that it requires a larger diametrical space as compared with the spline type clutch used in the structure of Akutagawa et al.
In the driving arrangement by Akutagawa et al, a compact structure is obtained by locating the inter-axle differential gear mechanism and the front inter-wheel differential gear mechanism on the front wheel axle and housing in the clutch housing Further, the spline type clutch is provided at a side of the inter-wheel differential gear mechanism adjacent to the engine. It will therefore be understood that in this arrangement there will be no sufficient space for accommodating the multiple disc type clutch in the place of the spline type clutch
The aforementioned U.S. patent to C. Hill teaches to locate the multiple disc type clutch between the planetary gear type inter-axle differential gear mechanism and a bevel gear which constitutes a transfer gear. The U.S patent to C. Hill et al also show a similar concept. This concept cannot be advantageously applied to the arrangement of Akutagawa et al because the transfer gear must be sidewardly displaced with respect to the inter-axle differential gear mechanism in order to provide a space for locating the clutch and the will be produced a dead space around the clutch making the driving arrangement bulky as a whole. It should further be noted that in case where the clutch is located between the inter-axle differential gear mechanism and the transfer gear, it will become difficult to access the clutch for maintenance and repair so that it will become necessary to disassemble the unit for having an access to the clutch.
The aforementioned British patent discloses a similar concept with respect to the location of the multiple disc type clutch for the locking device of the inter-axle differential gear mechanism. Therefore, the aforementioned problems cannot be solved by the teachings in the British patent.